Prior art involving silicon resonant magnetometers suffer from poor sensitivity and moderate detection limits above 1 μT. In contrast, autonomous navigation systems that use position, navigation, and time units (PNTs) in conjunction with GPS applications need sensitive magnetometers wherein a minimum desired detection angle of 0.1° (in terms of resolving capability) translates to a minimum detectable magnetic field of under 50 nT.
Additionally, prior art silicon resonators are known to exhibit high temperature dependence, resulting in low stability of the measurement, whereas quartz resonators can be manufactured in various temperature-insensitive crystal cuts (AT cuts and SC cuts are preferred). Moreover, he high stability of co-fabricated MEMS quartz oscillators can be exploited to stabilize the measurement of the MEMS magnetometer. So a MEMS quartz oscillator may be advantageously disposed on the same substrate as the magnetometer disclosed herein.
The prior art includes: “Development of Miniature Magnetometers” Dennis K. Wickenden, Thomas J. Kistenmacher, Robert Osiander, Scott A. Ecelberger, R. Ben Givens, and John C. Murphy, Johns Hopkins APL Technical Digest, Vol. 18, Num. 2 (1997) 271.
This prior art achieves good detection sensitivity but utilizes a large (>4 mm) xylophone beam. High drive currents (>1A) prevent widespread use in portable and low power devices. The material of choice does not lend this prior art device to be easily integrated with electronics or other sensors. The present invention is based on a quartz MEMS fabrication process which is the subject of U.S. Pat. No. 7,830,074 identified above which has already demonstrated wafer-level integration with electronics.
The prior art also includes:    “A Resonant Micromachined Magnetic Field Sensor,” Behraad Bahreyni, and Cyrus Shafai, IEEE Sensors Journal, VOL. 7, NO. 9, SEPTEMBER 2007    “A resonant magnetic field microsensor with high quality factor at atmospheric pressure,” A L Herrera-May, P J Garcia-Ramirez, L A Aguilera-Cortes, J Martinez-Castillo, A Sauceda-Carvajal, L Garcia-Gonzalez, and E Figueras-Costa, J. Micromech. Microeng. 19 (2009) 015016 (11pp)    “Low Power 3-axis Lorentz Force Navigation Magnetometer,” M. J. Thompson, M. Li, and D. A. Horsley, MEMS 2011, Cancun, Mexico, Jan. 23-27, 2011.    “Micromechanical magnetometer using an all-silicon nonlinear torsional resonator,” D. Antonio, M. I. Dolz, and H. Pastoriza, Applied Physics Letters 95, 133505 2009.
The prior art mentioned above uses silicon resonant structures whose frequency shifts in the presence of a changing magnetic field. While silicon devices can easily be integrated with on-chip electronics, they are prone to large frequency-temperature drifts. Prior art often requires external and often separate drive and detection schemes which increases the complexities and noise. We design the drive and sense mechanisms directly onto the sensing structure enabling a more compact sensor footprint and reduced parasitics. Additionally, directly coupling between the drive and sense mechanisms of our invention greatly improves SNR.
In one aspect the present invention provides a magnetometer comprising a resonating structure which is naturally resonant in at least first and second resonant modes, a resonant frequency of the second mode being at least an order of magnitude greater than a resonant frequency of the first mode, the resonating structure having two sense electrodes disposed on opposing major surfaces of the resonating structure and having a conductive path formed as a loop, the loop being disposed near or at edges of the resonating structure and the two sense electrodes being formed inwardly of the edges of the resonating structure and also inwardly of said loop, first and second oscillator circuits, the first oscillator circuit being coupled to said loop for applying an oscillating current to said loop, the oscillating current having a frequency essentially equal to the resonant frequency of the first mode of the resonating structure, the second oscillator circuit being coupled to said sense electrodes, the second oscillator circuit oscillating with a fundamental frequency corresponding to the resonant frequency of the second mode of the resonating structure, the second oscillator also producing sidebands indicative of the magnetometer sensing an external magnetic field.
In another aspect the present invention provides a resonating structure formed of a beam of dielectric material, the beam of dielectric material being naturally resonant in at least first and second resonant modes, wherein the first resonant mode is a flexure mode and the second resonant mode is a thickness shear mode, a resonant frequency of the thickness shear mode being at least an order of magnitude greater than a resonant frequency of the flexure mode, the resonating structure having two sense electrodes disposed on opposing major surfaces of the resonating structure and having a conductive path formed as a loop on said beam of dielectric material, the loop being disposed near or at edges of the beam of dielectric material and the two sense electrodes being formed inwardly of the loop.
In yet another aspect the present invention provides a magnetometer comprising a resonating structure which is naturally resonant in at least first and second resonant modes, a resonant frequency of the second mode being at least an order of magnitude greater than a resonant frequency of the first mode, the resonating structure having two sense electrodes disposed on opposing major surfaces of the resonating structure and having a conductive path formed as a loop, the loop being disposed near or at edges of the resonating structure and the two sense electrodes being formed inwardly of the edges of the resonating structure and also inwardly of said loop, an oscillating device coupled to said loop for applying an oscillating current to said loop, the oscillating current having a frequency at least within a bandwidth of a flexure mode frequency of the resonating structure, and an oscillator sustaining circuit coupled to said sense electrodes, the oscillator sustaining circuit oscillating with a fundamental frequency corresponding to the resonant frequency of the second resonant mode of the resonating structure, the oscillator sustaining circuit also producing sidebands indicative of the magnetometer sensing an external magnetic field.
In still yet another aspect the present invention provides a method of sensing a magnetic field using a crystalline quartz resonator disposed in said magnetic field, the method including inducing acoustic coupling between a mechanical mode of oscillation of said resonator caused by the magnetic field to be sensed and a piezoelectric mode of oscillation induced by applying an AC voltage to sense electrodes disposed on opposing sides of the quartz resonator and applying an AC current to a loop conductor disposed on said crystalline quartz resonator which enables the crystalline quartz resonator to sense the magnetic field due to sidebands which occur in said AC voltage which sidebands are indicative of the sensing the magnetic field through a resulting Lorentz force.
In yet another aspect the present invention provides a method of sensing a magnetic field using a quartz resonator disposed in said magnetic field, the method comprising: applying an RF signal to sense electrodes disposed on opposing sides of the quartz resonator thereby inducing the quartz resonator to vibrate in a shear mode of vibration at a fundamental frequency of ft.s., applying an AC signal, having a substantially lower frequency fflex than the frequency ft.s. of the RF signal, the AC signal being applied to at least one loop electrode disposed on the quartz resonator, and disposing the quartz resonator with the AC signal applied to the at least one loop conductor and the RF signal applied to the sense electrodes in the magnetic field to be sensed, the magnetic field interacting with the AC signal in the at least one loop conductor to drive the quartz resonator in a flexure mode of oscillation at the frequency fflex, the flexure mode of oscillation of the quartz resonator acoustically coupling with the shear mode of vibration to induce one or more sidebands in the AC signal, the at least one sideband having an amplitude which is related at least in part to an intensity of the magnetic field sensed by the quartz resonator.